Connecting method of optical function devices, and optical apparatus

ABSTRACT

The present invention aims at providing a connecting method capable of suppressing an influence of stray light for a plurality of optical function devices formed on the same substrate, and an optical apparatus applied with the control method. To this end, in the connecting method of optical function devices according to the present invention, the plurality of optical function devices formed on the same substrate are cascade connected so that both ends of an optical path passing through the plurality of optical function devices are positioned on the same end face of the substrate. According to such a connecting method, it becomes possible to effectively suppress a leakage of stray light from an optical input side to an optical output side.

BACKGROUND OF THE INVENTION

[0001] (1). Field of the Invention

[0002] The present invention relates to a technique for connecting aplurality of optical function devices formed on the same substrate, inparticular, to a connecting method capable of suppressing a leakage ofstray light from an optical input side to an optical output side, and anoptical apparatus applied with the connecting method.

[0003] (2). Related Art

[0004] There have been demanded developments of optical communicationsystems and optical signal processing systems capable of constructingnetworks of large capacities and ultra-long distance with an explosiveincrease of IP data communication demand. In a transmission systemadopting a wavelength-division multiplexing (WDM) transmission as abasic technique thereof, it is possible to realize the large capacitytransmission and easily perform the division-multiplication with thewavelength as a unit, so that the construction of flexible opticalnetworks that performs multiplication-division of different kinds ofservices at optical levels, such as, optical cross-connecting (OXC),optical add/drop multiplexing (OADM) and the like. Therefore, thedevelopment and manufacturing of transmission apparatus and signalprocessing apparatus using the above system have been remarkably made.

[0005] In these apparatuses, there are utilized many optical functiondevices, such as, an optical switch having functions for switchingON/OFF of light, for attenuating the light, for switching to 1×n, or thelike, a wavelength filter that separates a signal light for eachwavelength, or the like. Specifically, the optical switch (including anattenuator) is used, for example, for adjusting the levels of respectivewavelengths at a wavelength division multiplexing section on the sendingside, for ALC controlling by an optical amplifier, for wavelengthswitching in the OXC and OADM, for switching ON/OFF of light and thelike. Further, the optical filter is used, for example, for wavelengthswitching in the OXC and OADM, for separating the respective wavelengthson the receiving side, for cutting off ASE light and the like.

[0006] By forming these optical function devices on a substrate made ofSiO₂, LiNbO₃ and the like, it becomes possible to achieve the highfunctions, down-sizing, integration, reduction of electricity, andreduction of cost. The plurality of optical function devices integratedon the substrate are used individually in parallel with one another, forexample, as shown in FIG. 17, or are cascade connected in a multi-stagedstructure to be used, for example, as shown in FIG. 18A and FIG. 18B, soas to achieve the respective functions thereof. Thus, in a case wherethe respective optical function devices are used individually inparallel with one another, an effect owing to the integration becomeslarge. Further, in a case where the respective optical function devicesare cascade connected in a multi-staged structure to be used, it becomespossible to achieve the improvement of extinction ratio, if the opticalfunction devices are optical switches, while if the optical functiondevices are optical filters, such as, acousto-optic tunable filters(AOTFs), it becomes possible to achieve the narrow transmission band,the improvement of suppression ratio between other channels, and theimprovement of extinction ratio when used as notch filters. Moreover, ifdevices having different functions from one another are cascadeconnected in a multi-staged structure to be used, it becomes possible toachieve a high function and the like.

[0007] However, in a case where a plurality of optical function devicesintegrated on the substrate are connected to be used, most of the inputlight from a substrate input section passes through the optical functiondevices, however, as shown by an arrow in dotted line in FIG. 17 andFIGS. 18A and 18B, a part of the input light is emitted into thesubstrate, and bypasses the optical function devices as a stray light,to be coupled to an output section. The coupling of this stray light tothe output section causes deterioration of extinction ratio, in a casewhere the optical function devices are optical switches. Further, in acase where the optical function devices are optical filters, such asAOTFs and the like, the coupling of this stray light to the outputsection causes deterioration of suppression ratio between other channelsor deterioration of extinction ratio at the time when the AOTFs are usedas notch filters.

[0008] Specifically, as one example, the consideration is made on anoptical switch having characteristics in which an optical insertion lossis 10 dB and an extinction ratio at an optical function portion is 40dB, as shown in a solid line in FIG. 19, in a case where a leveldifference of stray light to an input light is 40 dB, the extinctionratio of the optical switch is about 30 dB. In a case where the leveldifference of stray light to an input light is 50 dB, the extinctionratio is about 37 dB. Thus, the extinction ratio of the optical switchis restricted by an influence of stray light. If a required value ofextinction ratio of the optical switch is assumed to be 40 dB, therequired value cannot be achieved even if the level difference of straylight to the input light is 50 dB or more. Therefore, it is necessary tomake the stray light level negligibly smaller compared to the extinctionratio at the optical function portion.

SUMMARY OF THE INVENTION

[0009] The present invention has been achieved in view of the aboveproblems, and an object of the present invention is to provide aconnecting method capable of suppressing an influence of stray light ina plurality of optical function devices formed on the same substrate,and an optical apparatus applied with the connecting method.

[0010] In order to achieve the above object, one aspect of a connectingmethod of the present invention, for connecting a plurality of opticalfunction devices formed on the same substrate, is constituted such thatthe plurality of optical function devices are cascade connected, so thatboth ends of an optical path passing through the plurality of opticalfunction devices are positioned on the same end face of the substrate.

[0011] Further, one aspect of an optical apparatus according to thepresent invention comprises a plurality of optical function devicesformed on the same substrate, and a cascade connecting section thatcascade connects the plurality of optical function devices so that bothends of an optical path passing through the plurality of opticalfunction devices are positioned on the same end face of the substrate.

[0012] According to the connecting method and the optical apparatus asdescribed above, a light is input/output to/from the same end face ofthe substrate, for the plurality of optical function devices cascadeconnected. Thus, there is reduced the rate that a stray light generatedon an optical input side bypasses the respective optical functiondevices to be coupled to the light being propagated within the opticalpath on an optical output side. Thereby, it becomes possible to suppresseffectively a leakage phenomenon of stray light from the optical inputside to the optical output side.

[0013] The other aspect of the connecting method according to thepresent invention, connecting a plurality of optical function devicesformed on the same substrate, is constituted such that the plurality ofoptical function devices are connected in parallel, so that lightinput/output directions of optical function devices adjacent to eachother on the substrate are opposite to each other.

[0014] The other aspect of optical apparatus according to the presentinvention comprises a plurality of optical function devices formed onthe same substrate, and a parallel connecting section that connects theplurality of optical function devices in parallel, so that lightinput/output directions of optical function devices adjacent to eachother on the substrate are opposite to each other.

[0015] According to the connecting method and the optical apparatus asdescribed above, since the propagation directions of lights input/outputto/from the optical function devices adjacent to each other are oppositeto each other, for the plurality of optical function devices connectedin parallel, there is reduced the rate that a stray light generated onthe optical input side of the adjacent optical function device iscoupled to the light being propagated within the output side opticalpath of the own optical function device. Thereby, it becomes possible tosuppress effectively a leakage phenomenon of stray light between opticalfunction devices adjacent to each other.

[0016] Further objects, features and advantages of the present inventionwill become more apparent from the following description of preferredembodiments when read in conjunction with the accompanying drawings.

BRIEF EXPLANATION OF THE DRAWINGS

[0017]FIG. 1 is a block diagram showing a first embodiment of an opticalapparatus according to the present invention;

[0018]FIG. 2 is a block diagram showing another constitutional exampleof an optical apparatus in the first embodiment;

[0019]FIG. 3 is a block diagram showing a second embodiment of anoptical apparatus according to the present invention;

[0020]FIG. 4 is a diagram showing a third embodiment of an opticalapparatus according to the present invention;

[0021]FIG. 5 is a block diagram showing a fourth embodiment of anoptical apparatus according to the present invention;

[0022]FIG. 6 is a block diagram showing a fifth embodiment of an opticalapparatus according to the present invention;

[0023]FIG. 7 is a plan view showing a constitution of rejection typeoptical filter as a specific example of an optical apparatus accordingto the present invention;

[0024]FIG. 8 is a diagram for explaining the cross-connection ofconnecting optical paths in the rejection type optical filter of FIG. 7;

[0025]FIG. 9 is a diagram showing one example of the end face shape of asubstrate in the rejection type optical filter of FIG. 7;

[0026]FIG. 10 is a diagram showing one example of fiber array structureconnected to the substrate end face in the rejection type optical filterof FIG. 7;

[0027]FIG. 11 is a diagram for explaining inter-polarization-modeinterference of a polarization-preserving fiber;

[0028]FIG. 12 is a diagram for explaining the selected wavelengthDoppler shift in AOTF;

[0029]FIG. 13 is a conceptual diagram for explaining filtercharacteristics of an optical filter of rejection type, in which FIG.13A shows ideal filter characteristics, FIG. 13B shows filtercharacteristics of when the selected wavelengths are coincident with oneanother in a multi-staged structure, and FIG. 13C shows filtercharacteristics of when the selected wavelengths are different from oneanother;

[0030]FIG. 14 is a diagram for explaining the deviation of selectedwavelengths inherent to the substrate on which three-staged AOTFs areintegrated, in which FIG. 14A to FIG. 14C are exemplary diagrams ofwavelength deviation patterns, and FIG. 14D is a diagram showing atypical wavelength deviation pattern;

[0031]FIG. 15 is a schematic view arranging optimum connectionrelationships in view of an influence of selected wavelength Dopplershift and the like, according to the wavelength deviation patterns inFIG. 14;

[0032]FIG. 16 is a diagram showing relationships among the selectedwavelengths at respective stages set in the rejection type opticalfilter in FIG. 7;

[0033]FIG. 17 is a diagram showing one example in which the opticalfunction devices on the same substrate are connected in parallel by aconventional connecting method;

[0034]FIG. 18 is a diagram showing one example in which the opticalfunction devices on the same substrate are cascade connected by aconventional connecting method, in which FIG. 18A shows a two-stagedstructure and FIG. 18B shows a three-staged structure; and

[0035]FIG. 19 is diagram for explaining an influence by a stray light ina conventional connecting method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Embodiments of the present invention will be described based onthe drawings.

[0037]FIG. 1 is a block diagram showing a first embodiment of an opticalapparatus applied with a connecting method of optical function devicesaccording to the present invention.

[0038] In FIG. 1, the optical apparatus in the first embodiment has aconstitution in which, for two optical function devices 11 and 12 formedon two optical waveguides 21 and 22 within the same substrate 1A,respectively, an input optical path 2 _(IN) is connected to one end 21 aof the optical waveguide 21, while an output optical path 2 _(OUT) isconnected to one end 22 a of the optical waveguide 22, positioned on theface same as the end face of the substrate 1A on which the one end 21 aof the optical waveguide 21 is positioned, and the other end 21 b of theoptical waveguide 21 is connected to the other end 22 b of the opticalwaveguide 22 by a connecting optical path 2 ₁₂ so as to cascade connectthe two optical function devices 11 and 12.

[0039] The substrate 1A is constituted such that two optical waveguides21 and 22 substantially parallel with each other, and two opticalfunction devices 11 and 12, are formed on a substrate material made of,for example, SiO₂ or LiNbO₃, by applying a required treatment on thesubstrate material. The optical function devices 11 and 12 may be knownoptical function devices, such as, optical switch, optical attenuator,optical filter and the like. Or, any of devices of optical waveguidetype or non-optical waveguide type may be used. Further, the opticalfunction devices 11 and 12 may be provided with same function or withdifferent functions, respectively. Note, an example of specificconstitution will be described later.

[0040] The input optical path 2 _(IN), output optical path 2 _(OUT), andconnecting optical path 2 ₁₂ are known optical paths, such as opticalfiber or optical waveguide, respectively. An input light to the presentoptical apparatus is propagated through the input optical path 2 _(IN),to be input to the one end 21 a of the optical waveguide 21, and passesthrough the optical waveguide 21 to be given to the optical functiondevice 11. Then, the light passed through the optical function device 11is output from the other end 21 b of the optical waveguide 21, and ispropagated through the connecting optical path 2 ₁₂ to be input to theother end 22 b of the optical waveguide 22 on the substrate 1A, andfurther passes through the optical waveguide 22 to be given to theoptical function device 12. The light passed through the opticalfunction device 12 is output from the one end 22 a of the opticalwaveguide 22, and is propagated through the output optical path 2_(OUT), to be output as an output light of the present opticalapparatus.

[0041] In the optical apparatus having the above constitution, most ofthe input light given to the one end 21 a of the optical waveguide 21 onthe substrate 1A is propagated through the optical waveguide 21 to besent to the optical function device 11, however, as shown by an arrow indotted line in FIG. 1, a part of the input light is emitted into thesubstrate as a stray light S. This stray light S bypasses the opticalfunction device11, and is propagated towards the end face opposite tothe optical input side of the substrate 1A. However, in the presentoptical apparatus, since the output optical path 2 _(OUT) is connectedto the one end 22 a of the optical waveguide 22, positioned on the facesame as the end face to which the input optical path 2 _(IN) of thesubstrate 1A is connected, the stray light S is hardly to be coupled tothe light being propagated near the one end 22 a within the opticalwaveguide 22. Therefore, it becomes possible to suppress effectively aleakage phenomenon of the stray light S from the optical input side tothe optical output side on the substrate 1A. Specifically, in thepresent optical apparatus, in a case where the optical function devices11 and 12 are optical switches and the like, it becomes possible toachieve the improvement of extinction ratio. Further, in a case wherethe optical function devices 11 and 12 are optical filters and the like,it becomes possible to the improvement of suppression ratio betweenother channels or the improvement of extinction ratio at the time whenthe filters are used as notch filters.

[0042] In this way, according to the present optical apparatus, itbecomes possible to sufficiently effect individual characteristics ofthe two optical function devices 11 and 12 on the substrate 1A. Thus, itbecomes useful for miniaturization, low-cost of optical apparatus tomake the optical devices integrated on the same substrate applicable tovarious optical transmission apparatuses.

[0043] In the above first embodiment, there has been described the casewhere the other ends 21 b and 22 b of the optical waveguides 21 and 22are directly connected to each other by the connecting optical path 2₁₂. However, the present invention is not limited thereto, and theconstitution may be such that another optical device 3 is disposed onthe connecting optical path 2 ₁₂, as shown in FIG. 2, for example. As aspecific example of another optical device 3, an optical amplifier maybe used. However, the optical device is not limited to the opticalamplifier.

[0044] Next, there will be described a second embodiment of an opticalapparatus according to the present invention.

[0045]FIG. 3 is a block diagram showing the second embodiment of anoptical apparatus applied with a connecting method of optical functiondevices according to the present invention. Same components as those inthe first embodiment are denoted by the same reference numerals and thedescriptions thereof shall be omitted. Same rules shall be applied tothe other embodiments.

[0046] In FIG. 3, the optical apparatus of the second embodimentcorresponds to a case where three optical function devices integrated onthe same substrate are cascade connected. Specifically, for threeoptical function devices 11, 12 and 13 that are formed on three opticalwaveguides 21, 22 and 23 within a substrate 1B, respectively, the inputoptical path 2 _(IN) is connected to the one end 21 a of the opticalwaveguide 21, while the output optical path 2 _(OUT) is connected to oneend 23 a of the optical waveguide 23, positioned on the face same as theend face of the substrate 1B on which the one end 21 a of the opticalwaveguide 21 is positioned. Further, the one end 22 a of the opticalwaveguide 22 is connected to the other end 23 b of the optical waveguide23 by a connecting optical path 2 ₂₃, and the other end 21 b of theoptical waveguide 21 is connected to the other end 22 b of the opticalwaveguide 22 by the connecting optical path 2 ₁₂. Thereby, the presentoptical apparatus has the constitution in which three optical functiondevices 11 to 13 are cascade connected.

[0047] The substrate 1B is constituted such that three opticalwaveguides 21 to 23 substantially parallel with one another, and threeoptical function devices 11 to 13, are formed on a required substratematerial in the same manner as in the first embodiment. The opticalfunction devices 11 to 13 are the same as the optical function devicesdescribed in the first embodiment.

[0048] The input light to the present optical apparatus is propagatedthrough the input optical path 2 _(IN), to be input to the one end 21 aof the optical waveguide 21, and passes through the optical waveguide 21to be given to the optical function device 11. The light passed throughthe optical function device 11 is output from the other end 21 b of theoptical waveguide 21, and is propagated through the connecting opticalpath 2 ₁₂ to be input to the other end 22 b of the optical waveguide 22on the substrate 1B, and further passes through the optical waveguide 22to be given to the optical function device 12. The light passed throughthe optical function device 12 is output from the one end 22 a of theoptical waveguide 22, and is propagated through the connecting opticalpath 2 ₂₃ to be input to the other end 23 b of the optical waveguide 23on the substrate 1B, and further passes through the optical waveguide 23to be given to the optical function device 13. The light passed throughthe optical function device 13 is output from the one end 23 a of theoptical waveguide 23, and is propagated through the output optical path2 _(OUT), to be output as an output light of the present opticalapparatus.

[0049] In the optical apparatus having the above constitution, as in thefirst embodiment, since the output optical path 2 _(OUT) is connected tothe one end 23 a of the optical waveguide 23, positioned on the facesame as the end face to which the input optical path 2 _(IN) of thesubstrate 1B is connected, the stray light S from the optical input sideshown by a dotted line in FIG. 3 is hardly to be coupled to the lightbeing propagated near the one end 23 a within the optical waveguide 23.Therefore, it becomes possible to suppress effectively a leakagephenomenon of the stray light S from the optical input side to theoptical output side on the substrate 1B, so that the individualcharacteristics of three optical function devices 11 to 13 on thesubstrate 1B can sufficiently be effected.

[0050] In the above first or second embodiment, there has been describedthe case where two or three optical function devices on the samesubstrate are cascade connected. However, the connecting methodaccording to the present invention can also be applied to a case wherefour or more optical function devices on the same substrate are cascadeconnected.

[0051] Next, there will be described a third embodiment of an opticalapparatus according to the present invention.

[0052]FIG. 4 is a block diagram showing the third embodiment of anoptical apparatus applied with a connecting method of optical functiondevices according to the present invention.

[0053] In FIG. 4, the optical apparatus of the third embodimentcorresponds to, for example, a case where three optical function devicesintegrated on the same substrate are cascade connected in a loop. Inparticular, herein, there is shown a constitution suitable for a casewhere at least one of the optical function devices has the polarizationdependence. Specifically, optical input and output portions of thesubstrate 1B that has the same constitution as in the second embodimentare connected to the input optical path 2 _(IN) and the output opticalpath 2 _(OUT) by using an optical circulator 4, a polarization beamsplitter (PBS) 5 and connecting optical paths 2 _(A), 2 _(B), 2 _(C), sothat three optical function devices 11 to 13 are cascade connected in aloop.

[0054] The optical circulator 4 is a typical optical component thatincludes at least three ports 4 a, 4 b and 4 c, and transmits the lightonly in a direction from the port 4 a to port 4 b, from the port 4 b toport 4 c, and from the port 4 c to port 4 a. This optical circulator 4is connected with the input optical path 2 _(IN), the connecting opticalpath 2 _(A) to be connected to a PBS 5, and the output optical path 2_(OUT), to the port 4 a, port 4 b, and port 4 c, respectively.

[0055] The PBS 5 splits an input light sent from the port 4 b of theoptical circulator 4 via the connecting optical path 2 _(A) into twopolarization lights with polarization planes thereof being orthogonal toeach other, to output one of the two polarization lights to one end ofthe connecting optical path 2 _(B), while outputting the otherpolarization light to one end of the connecting optical path 2 _(C). Theother end of the connecting optical path 2 _(B) is connected to the oneend 21 a of the optical waveguide 21 on the substrate 1B, and the otherend of the connecting optical path 2 _(C) is connected to the one end 23a of the optical waveguide 23 on the substrate 1B. Further, the PBS 5multiplexes two polarization lights with the polarization planes beingcrossing each other sent from the substrate 1B via the connectingoptical paths 2 _(A) and 2 _(B), to output the multiplexed polarizationlight to the connecting optical path 2 _(A).

[0056] Note, the constitution that the one end 22 a of the opticalwaveguide 22 on the substrate 1B is connected to the other end 23 b ofthe optical waveguide 23 via the connecting optical path 2 ₂₃, and theother end 21 b of the optical waveguide 21 is connected to the other end22 b of the optical waveguide 22 via the connecting optical path 2 ₁₂,is same as for the second embodiment.

[0057] In the optical apparatus having the above constitution, the inputlight being propagated through the input optical path 2 _(IN) is sent tothe PBS 5 via the optical circulator 4 and the connecting optical path2_(A), and is split into two polarization lights crossing each other tobe output to the connecting optical paths 2 _(B) and 2 _(C),respectively.

[0058] One polarization light output to the connecting optical path 2_(B) from the PBS 5 is input to the one end 21 a of the opticalwaveguide 21 on the substrate 1B, and passes through the opticalwaveguide 21 to be given to the optical function device 11, and thenpasses through the connecting optical path 2 ₁₂ and the opticalwaveguide 22 to be given to the optical function device 12, and further,passes through the connecting optical path 2 ₂₃ and the opticalwaveguide 23 to be given to the optical function device 13. Then, thepolarization light passed through the optical function device 13 isoutput to the connecting optical path 2 _(C) from the one end 23 a ofthe optical waveguide 23, to be returned the PBS 5.

[0059] Further, the other polarization light output to the connectingoptical path 2 _(C) from the PBS 5 passes through the respective opticalfunction devices on the substrate 1B in a direction opposite to thedirection of the one polarization light. That is, the other polarizationlight is input to the one end 23 a of the optical waveguide 23 on thesubstrate 1B, and passes through the optical waveguide 23 to be given tothe optical function device 13, and then passes through the connectingoptical path 2 ₂₃ and the optical waveguide 22 to be given to theoptical function device 12, and further, passes through the connectingoptical path 2 ₁₂ and the optical waveguide 21 to be given to theoptical function device 11. Then, the polarization light passed throughthe optical function device 11 is output to the connecting optical path2 _(B) from the one end 21 a of the optical waveguide 21, to be returnedthe PBS 5.

[0060] As described above, when the respective polarization lights splitby the PBS 5 are propagated in bi-directions through three opticalfunction devices cascade connected on the substrate 1B, the stray lightsS generated in the one ends 21 a and 23 a of the optical waveguides 21and 23 as exemplarily shown by arrows in dotted lines in FIG. 4, arepropagated towards the end face opposite to the optical input side ofthe substrate 1B. However, also in the present optical apparatus, as inthe second embodiment, since the respective connecting optical paths 2_(B) and 2 _(C) are connected to the one ends 21 a and 23 a of theoptical waveguides 21 and 23, positioned on the same end face of thesubstrate 1B, the respective stray lights S are hardly to be coupled tothe polarization lights being propagated near the one ends 21 a and 23 awithin the optical waveguides 21 and 23.

[0061] Then, the respective polarization lights returned to the PBS 5via the connecting optical paths 2 _(B) and 2 _(C) are multiplexed bythe PBS, and then sent to the optical circulator 4 via the connectingoptical path 2 _(A), to be output to the output optical path 2 _(OUT)after passing through from the port 4 b to the port 4 c.

[0062] As described above, also in the optical apparatus of the thirdembodiment in which three optical function devices are cascadeconnected, it is possible to suppress a leakage phenomenon of the straylights S being propagated within the substrate 1B, to the optical outputside, so that individual characteristics of three optical functiondevices 11 to 13 on the substrate 1B can be sufficiently effected.

[0063] Note, in the third embodiment, three optical function devices onthe same substrate are cascade connected in a loop by applying theconnecting method according to the present invention. However, it isalso possible to cascade connect in a loop two optical devices, or fouror more optical devices on the same substrate by applying the connectingmethod according to the present invention. Moreover, there has beendescribed the case where the optical function device has thepolarization dependence. However, even when the optical function devicedoes not have the polarization dependence, surely it is possible toperform the cascade loop connection by applying the connecting method ofthe present invention.

[0064] Next, there will be described a fourth embodiment of an opticalapparatus according to the present invention.

[0065]FIG. 5 is a block diagram showing the fourth embodiment of anoptical apparatus applied with a connecting method of optical functiondevices according to the present invention.

[0066] In FIG. 5, the optical apparatus of the fourth embodiment is anapplication example for the optical apparatus of the second embodiment,in which a plurality of (herein, two) three-staged cascade connectionconfiguration. Specifically, for six optical function devices 11 to 13and 11′ to 13′ formed on six optical waveguides 21 to 23 and 21′ to 23′within the same substrate 1C, respectively, input optical paths 2 _(IN)and 2 _(IN), are connected to one ends 21 a and 21 a′ of the opticalwaveguides 21 and 21′, respectively, and output optical paths 2 _(OUT)and 2 _(OUT′) are connected to one ends 23 a and 23 a′ of the opticalwaveguides 23 and 23′, positioned on the face same as the end face ofthe substrate 1C on which the one ends 21 a and 21 a′ of the opticalwaveguides 21 and 21′ are positioned. Further, the one ends 22 a and 22a′ of the optical waveguides 22 and 22′ are connected to the other ends23 b and 23 b′ of the optical waveguides 23 and 23′ by connectingoptical paths 2 ₂₃ and 2 ₂₃′, and the other ends 21 b and 21 b′ of theoptical waveguides 21 and 21′ are connected to the other ends 22 b and22 b′ of the optical waveguides 22 and 22′ by connecting optical paths 2₁₂ and 2 ₁₂′. Thereby, the present optical apparatus has theconstitution in which six optical function devices 11 to 13 and 11′ to13′ are cascade connected in two groups.

[0067] In the optical apparatus having the above constitution, for thethree-staged structure obtained by cascade connecting the opticalfunction devices 11 to 13 and the three-staged structure obtained bycascade connecting the optical function devices 11′ to 13′, as in thesecond embodiment, the input lights from the input optical paths passthrough the three-staged optical function devices sequentially, to besent to the optical output optical paths as output lights. Since theinput optical paths 2 _(IN) and 2 _(IN)′ and output optical paths 2_(OUT) and 2 _(OUT)′ of the respective three staged structures areconnected to one ends of the optical waveguides 21 and 23, and 21′ and23′, positioned on the same end face of the substrate 1C, the respectivestray lights S from the respective optical input sides exemplarily shownby arrows in dotted lines of FIG. 4 are hardly to be coupled to thelights being propagated near the one ends 23 a and 23 a′ within theoptical waveguides 23 and 23′. Thereby, also in the optical apparatushaving two groups of cascade connection configuration on the samesubstrate 1C, it is possible to suppress effectively a leakage of thestray lights S.

[0068] In the above fourth embodiments, there has been described thecase where two groups of three-stages cascade connection configurationare provided on the same substrate. However, the number of groups andthe number of stages of the cascade connection configuration on the samesubstrate in the present invention are not limited to the above example.Further, the cascade loop configuration as shown in the third embodimentcan also be applied as in the same manner as described above.

[0069] Next, there will be described a fifth embodiment of an opticalapparatus according to the present invention.

[0070]FIG. 6 is a block diagram showing the fifth embodiment of anoptical apparatus applied with the connecting method of optical functiondevices according to the present invention.

[0071] In FIG. 6, the optical apparatus in the fifth embodimentcorresponds to a case where optical function devices of n in numberintegrated on the same substrate are individually used in parallel.Specifically, for optical function devices 10 ₁ to 10 _(n) of n innumber formed on substantially parallel optical waveguides 20 ₁ to 20_(n) of n in number within the same substrate 1D, the optical waveguides20 ₁ to 20 _(n) are connected to input optical paths 2 _(IN1) to 2_(INn), and to output optical paths 2 _(OUT1) to 2 _(OUTn), so thatinput/output directions of lights to/from optical function devicesadjacent to each other are opposite to each other.

[0072] Herein, if one end 20 a ₁ of the optical waveguide 20 ₁ isconnected to the input optical path 2 _(IN1) and the other end 20 b ₁ isconnected to the output optical path 2 _(OUT1), one end 20 a ₂ of theoptical waveguide 20 ₂ adjacent to the optical waveguide 20, isconnected to the output optical path 2 _(OUT2) and the other end 20 b ₂is connected to the input optical path 2 _(IN2). Further, one end 20 a ₃of the optical waveguide 20 ₃ adjacent to the optical waveguide 20 ₂ isconnected to the input optical path 2 _(IN3) and the other end 20 b ₃ isconnected to the output optical path 2 _(OUT3). In the same manner asthe above, the connection is sequentially performed so that theinput/output directions of lights to/from the respective opticalwaveguides 20 ₄ to 20 _(n) are opposite to each other. Thus, theinput/output directions of lights to/from the optical function devicesadjacent to each other are opposite to each other.

[0073] In this way, for example, most of the input light given to theone end 20 a ₁ of the optical waveguide 20 ₁ from the input optical path2 _(IN1) is propagated through the optical waveguide 20 ₁ to be sent tothe optical function device 10 ₁, however, as shown by an arrow indotted line in FIG. 6, a part of the input light is emitted into thesubstrate as a stray light S. This stray light S is propagated towardsthe end face opposite to the optical input side of the substrate 1D.However, since the adjacent optical waveguide 20 ₂ is connected to theinput optical path 2 _(IN2) at the other end 20 b ₁ and to the outputoptical path 2 _(OUT2) at the one end 20 a ₂, there is sufficientlyreduced the rate that the stray light S from the one end 20 a ₁ of theoptical waveguide 20 ₁ is coupled to the light being propagated near theone end 20 a ₂ of the optical waveguide 20 ₂, compared to theconventional connection configuration in which the light is input/outputin the same direction to/from the adjacent optical function devices.Further, although the stray light S is generated by the input lightgiven to the other end 20 b ₂ of the waveguide 20 ₂, there issufficiently reduced the rate that the stray light S is coupled to thelight being propagated near the one end 20 b ₁ of the adjacent opticalwaveguide 20 ₁.

[0074] The above connection relationships of input and output opticalpaths to the stray light S are established for all the optical functiondevices adjacent to one another on the substrate 1D. Therefore,according to the present optical apparatus, it becomes possible tosuppress effectively a leakage phenomenon of the stray light S so as tosufficiently effect the individual characteristics of the opticalfunction devices of n in number to be used in parallel.

[0075] Next, there will be a specific embodiment of an optical apparatusapplied with a connecting method of optical function devices accordingto the present invention. In the following, the consideration is made,as an example, on a rejection type optical filter embodied the opticalapparatus of the third embodiment as a basic constitution.

[0076]FIG. 7 is a plan view showing the constitution of the rejectiontype optical filter.

[0077] In the rejection type optical filter shown in FIG. 7, forexample, acousto-optic tunable filters (AOTF) are adopted as threeoptical function devices 11 to 13 formed on the same substrate 1.Optical input and output portions of the substrate 1 are connected tothe input optical path 2 _(IN) and the output optical path 2 _(OUT) byusing the optical circulator 4, the polarization beam splitter (PBS) 5,a polarization rotating section 6 and the connecting optical paths 2_(A), 2 _(B), 2 _(C), so that the three AOTFs on the substrate 1 arecascade connected in a loop.

[0078] The substrate 1 is constituted such that three optical waveguides21, 22 and 23 substantially parallel with one another are formed on asubstrate material made of, for example, LiNbO₃. The respective opticalwaveguides 21 to 23 are provided with polarization beam splitters (PBS)31 a, 31 b, 32 a, 32 b, 33 a and 33 b, respectively, at both endportions thereof. Also, the substrate 1 is formed with interdigitaltransducers (IDT) 41, 42 and 43, and SAW guides 51, 52 and 53,corresponding to the optical waveguides 21 to 23, respectively.

[0079] As the respective PBSs 31 a, 31 b, 32 a, 32 b, 33 a and 33 b, itis possible to use, for example, PBSs of crossing waveguide type and thelike. Here, input and output ports of the PBSs positioned at thecrossing sides of the crossing waveguides are connected to the opticalwaveguides, respectively, so that the respective PBSs are constituted tobe of TE mode transmission type.

[0080] The respective IDTs 41 to 43 are applied commonly with a signalof required frequency f generated by an RF signal generating circuit 40,to generate surface acoustic waves (SAW), respectively, on the substrate1. Note, as will be described later, positions of the respective IDTs 41to 43 are preferably set such that relationships between the propagationdirections of SAWs and the propagation directions of lights within thecorresponding optical waveguides are those taking into account ofinfluences of selected wavelength Doppler shift and the like.

[0081] The SAW guides 51 to 53 are those for propagating respective SAWsgenerated at the IDTs 41 to 43 through the optical waveguides 21 to 23,respectively. Here, a case is shown where, for example, SAW guides ofdirectional coupling type formed in required shape by Ti diffusion areused, as the SAW guides 51 to 53.

[0082] In the AOTF using the SAW guides of directional coupling type,SAWs generated at the IDTs are directionally coupled by the SAW guidesof required shape, so that SAWs most strongly interfere the light beingpropagated through the optical waveguide in the vicinity of the centerof mode conversion area. Thus, it is possible to achieve the suppressionof side lobe level in the filter characteristics of AOTF. Note, in theSAW guides shown in FIG. 7, curving shapes are adopted in order todirectionally couple SAWs in accordance with a further desired function.In this way, it becomes possible to suppress further effectively theside lobe level.

[0083] Here, the case is shown where the AOTF using the SAW guides ofdirectional coupling type is used. However, the present invention is notlimited thereto, and it is possible to use AOTF and the like formed withSAW guides of thin film type on the optical waveguides. Further, for theAOTF using the SAW guides of thin film type, the arrangement may be suchthat the longitudinal direction of each SAW guide is inclined by arequired amount to the axial direction of the optical waveguide so thatthe propagation axis of SAW and the optical axis cross each other at aninclined angle. By adopting such an arrangement, the intensity ofsurface acoustic wave sensed by the light is weighted in thelongitudinal direction. Thus, it becomes possible to achieve thesuppression of side lobe level.

[0084] The optical circulator 4 and the PBS 5 are the same as those usedin the third embodiment. This optical circulator 4 is connected with theinput optical path 2 _(IN), the connecting optical path 2 _(A) to beconnected to a PBS 5, and the output optical path 2 _(OUT), to the port4 a, port 4 b, and port 4 c, respectively.

[0085] The connection of the PBS 5 and the substrate 1 is such that theother end of the connecting optical path 2 _(B), to which the onepolarization light split by the PBS 5 is input to the one end thereof,is connected to the PBS 31 a positioned on the optical waveguide 21 ofthe substrate 1. The other end of the connecting optical path 2 _(C), towhich the other polarization light split by the PBS 5 is input to theone end thereof, is connected to the PBS 32 a positioned on the opticalwaveguide 22 of the substrate 1. Also, herein, a polarization rotatingsection 6 is inserted onto the connecting optical path 2 _(C). Thepolarization rotating section 6 has a function for rotating thepolarization plane of the other polarization light split by the PBS 5 by90 degrees.

[0086] The PBS 31 b positioned on the optical waveguide 21 of thesubstrate 1 is connected to the PBS 33 b positioned on the opticalwaveguide 23 by the connecting optical path 2 ₁₃. Further, the PBS 32 bpositioned on the optical waveguide 22 of the substrate 1 is connectedto the PBS 33 a positioned on the end portion of the optical waveguide23 by the connecting optical path 2 ₂₃. Thus, three AOTFs for mainsignal on the substrate 1 are cascade connected in a loop between theinput optical path 2 _(IN) and the output optical path 2 _(OUT).

[0087] The connecting optical paths 2 _(B), 2 _(C), 2 ₁₃ and 2 ₂₃ arepolarization-preserving fibers, and here, for example, PANDA type fibersare used. However, the structure of polarization-preserving fiber is notlimited to the PANDA type fiber, and it is possible to adopt a knownstructured fiber. Further, each of the connecting optical paths 2 _(B),2 _(C), 2 ₁₃ and 2 ₂₃ is spliced by rotating the polarization axissubstantially by 90 degrees (cross-connection) in the vicinity of alongitudinal direction thereof as shown in FIG. 8, and suppresses aninfluence due to the deviation of polarization axis of when connectingan optical device having polarization dependence (PDL) by thepolarization-preserving fiber, as described later.

[0088] It is preferable that two end faces opposite to each other of thesubstrate 1 to which the respective connecting optical paths 2 _(B), 2_(C), 2 ₁₃ and 2 ₂₃ are connected, are inclined by required angles so asto reduce an influence of reflected light at the faces connected withthe respective optical paths, for example, as shown in FIG. 9. Also, itis preferable that the optical fibers to be connected to each of thesubstrate end faces are structured in a fiber array, for example, asshown in FIG. 10. Note, the optical fibers provided in parallel to therespective connecting optical paths 2 ₁₃ and 2 ₂₃ in FIG. 10, are forextracting the dropped lights and the like to be blocked from passingthrough by the AOTFs at respective stages. An arrangement for thepolarization axes of the polarization-preserving fibers within the fiberarray is desirable to be set, considering the symmetry with a fiberarray connected to the substrate end face on the opposite side, so thatthe kinds of the both side fiber arrays are the same.

[0089] The substrate 1 is provided with a monitoring section 100 thatmonitors a dropped light, and an RF signal controlling section 200 thatcontrols the operation of the RF signal generating circuit 40 based onthe monitoring result.

[0090] The monitoring section 100 comprises an optical isolator 101A anda light receiver 102A for monitoring a dropped light from the lightssequentially passing in one direction through the respective AOTFscascade loop connected to one another, an optical isolator 101B and alight receiver 102B for monitoring a dropped light from the lightssequentially passing in the other direction through the respective AOTFscascade loop connected to one another, and a circuit 103 that adds upoutput signals photo-electrically converted by the light receivers 102Aand 102B, to output a monitor signal M. Here, an input port of theoptical isolator 101A is connected to a TM mode output port of the PBS31 b on the substrate 1, while an input port of the optical isolator101B is connected to a TM mode output port of the PBS 32 b on thesubstrate 1. As described later, it is desirable to set a position formonitoring the dropped signal for the light in each direction to an AOTFstage wherein the selected wavelength (dropped wavelength) is positionedat the center of blocking band, considering an influence of dithering tobe given to the RF signal.

[0091] In the rejection type optical filter having the above mentionedconstitution, as in the third embodiment, the input light propagatedthrough the input optical path 2 _(IN) is sent to the PBS 5 via theoptical circulator 4 and the connecting optical path 2 _(A), and splitinto two polarization lights orthogonal to each other, to be output tothe connecting optical paths 2 _(B) and 2 _(C), respectively. Thepolarization light output to the connecting optical path 2 _(C) isrotated with polarization plane thereof by 90 degrees by thepolarization rotating section 6, to be aligned with the polarizationdirection of the polarization light output to the connecting opticalpath 2 _(B). Then, the respective polarization lights propagated throughthe connecting optical paths 2 _(B) and 2 _(C) are given to the PBSs 31a and 32 a on the substrate 1, respectively, as the TE mode lights.Note, in FIG. 7, the polarization directions of propagated lights areindicated together with the cross section of arrangement of polarizationaxes of the PANDA type fibers, so that the polarization directions ofpropagated lights at the respective portions on the optical pathscascade loop connected can be clearly understood.

[0092] The TE mode light given to the PBS 31 a passes therethrough andis propagated through the optical waveguide 21 toward the PBS 31 b. Atthis time, SAW generated as a result that the RF signal of frequency ffrom the RF signal generating circuit 40 is applied to the IDT 41, isguided along the optical waveguide 21 by the SAW guide 51, to bepropagated in the same direction (forward direction) as the propagatedlight within the optical waveguide 21. Due to the acousto-optic effectby this SAW, only the light of wavelength corresponding to the frequencyof SAW (selected wavelength) out of the TE mode light being propagatedwithin the optical waveguide 21, is mode converted into a TM mode light.Then, the lights of respective modes reach the PBS 31 b, the TE modelight of wavelengths different from the selected wavelength(non-selected wavelengths), that has not been mode converted, passesthrough the PBS 31 b to be output to the connecting optical path 2 ₁₃,while the mode converted TM mode light of selected wavelength isbranched by the PBS 31 b as a dropped light, to be sent to the opticalisolator 101A of the first monitoring section 100.

[0093] The TE mode light output to the connecting optical path 2 ₁₃passes through the PANDA type fiber that is spliced by 90 degrees in thevicinity of the center in the longitudinal direction, to be sent to thePBS 33 b on the optical waveguide 23. At this time, a periodicwavelength dependence loss or polarization mode dispersion (PMD) due tointer-polarization-mode interference caused in the PANDA type fiber, anda polarization dependence loss (PDL) caused in the PBS on the substrate1 and the like are offset in front of and behind the 90 degree splicepoint, to be suppressed.

[0094] Here, there will be described the inter-polarization-modeinterference caused within the optical paths of polarization-preservingtype.

[0095] As in the above rejection type optical filter, in a case where adevice such as PBS having PDL is positioned between the optical functiondevice and the polarization-preserving fiber, or a case where eachoptical device has PDL due to the connection between the optical devicesby the polarization-preserving fiber, it is an ideal to perform theconnection by completely coinciding the polarization axis (Fast axis,Slow axis) directions of the polarization-preserving fiber with the PBSdirection or the PDL direction. However, in the actual connection of thepolarization-preserving fiber with the optical devices, it is difficultto completely coincide the axis directions with each other and thus,certain axis deviation cannot be avoided.

[0096] If the axis deviation as mentioned above is caused, as shown inFIG. 11, the inter-polarization-mode interference of thepolarization-preserving fiber is caused, resulted in the periodicwavelength dependence loss in the transmission characteristics ofoptical devices. The period of this periodic wavelength dependence lossbecomes 1/τ, if a difference between the propagation times of Fast axisand Slow axis of the polarization-preserving fiber is τ. Such a periodicwavelength dependence loss due to the inter-polarization-modeinterference of the polarization-preserving fiber causes a change inlevel of transmission light in an optical filter of rejection typeaccording to the wavelength, to lead characteristic deterioration.

[0097] Therefore, in the present optical filter, by splicing the PANDAtype fiber by rotating the polarization axis thereof by 90 degrees inthe vicinity of the center of the connecting optical path in thelongitudinal direction, the respective directions of Fast axis and Slowaxis are switched in front of and behind the splicing point, so that theinfluence by the above mentioned periodic wavelength dependence loss,PMD or PDL are offset.

[0098] The TE mode light sent to the PBS 33 b on the substrate 1 passestherethrough and is propagated within the optical waveguide 23 towardthe PBS 33 a. At this time, SAW generated at the IDT 43 and guided bythe SAW guide 53 is propagated in a reverse direction to the propagatedlight within the optical waveguide 23. Due to the acoust-optic effect bythis SAW, only the light corresponding to the selected wavelength out ofthe TE mode light being propagated through the optical waveguide 23 ismode converted into a TM mode light. Then, when the lights of respectivemodes reach the PBS 33 a, the TE mode light of non-selected wavelengths,that has not been mode converted, passes through the PBS 33 a to beoutput to the connecting optical path 2 ₂₃, while the mode converted TMmode light of selected wavelength is branched by the PBS 33 a.

[0099] The TE mode light output to the connecting optical path 2 ₂₃ issent to the PBS 32 b on the optical waveguide 22 while the periodicwavelength dependence loss and the like thereof being suppressed bypassing the PANDA type fiber that is spliced by 90 degrees, in the samemanner as when passed through the connecting optical path 2 ₁₃.

[0100] The TE mode light sent to the PBS 32 b passes therethrough and ispropagated within the optical waveguide 22 toward the PBS 32 a. At thistime, SAW generated at the IDT 42 and guided by the SAW guide 52 ispropagated in a forward direction to the propagated light within theoptical waveguide 22. Due to the acoust-optic effect by this SAW, onlythe light corresponding to the selected wavelength out of the TE modelight being propagated through the optical waveguide 22 is modeconverted into a TM mode light. The TE mode light of non-selectedwavelengths, that has not been mode converted, passes through the PBS 32a to be output to the connecting optical path 2 _(C), while the modeconverted TM mode light of selected wavelength is branched by the PBS 32a. The TE mode light output to the connecting optical path 2 _(C) isrotated with the polarization plane thereof by 90 degrees by thepolarization rotating section 6 on the connecting optical path 2 _(C)and then returned to the PBS 5.

[0101] The respective selected wavelengths to be mode converted at therespective optical waveguides 21 to 23 are slightly different from oneanother, due to the selected wavelength Doppler shift to be described inthe following, or inherent wavelength deviation caused by variations inmanufacturing process of the substrate 1, even in a constitution wherethe RF signal is applied commonly to the IDTs 41 to 43.

[0102] Here, the selected wavelength Doppler shift will be described.

[0103] The selected wavelength Doppler shift is a phenomenon in whichthe wavelengths of the light to be polarization mode converted becomedifferent from one another due to the acousto-optic effect, depending ona relationship between the propagation direction of light within theoptical waveguide and that of SAW transmitted along that opticalwaveguide. This phenomenon is caused by the same theory as that oftypically known Doppler shift, and in the above case, it can beconsidered that the wavelength (frequency) of SAW viewed from the lightis changed. Accordingly, for example, as shown in FIG. 12, if thepropagation direction of light is the same forward direction as thepropagation direction of SAW, the wavelength of SAW sensed by the lightbecomes longer. On the contrary, if the propagation direction of lightis the reverse direction to the propagation direction of SAW, thewavelength of SAW sensed by the light becomes shorter. The selectedwavelength λ in a case of influenced by such a Doppler shift, can berepresented by the following equation (1); $\begin{matrix}{\lambda = \frac{\lambda_{0}}{1 - {v/c}}} & (1)\end{matrix}$

[0104] wherein λ₀ is the selected wavelength in a case where SAW isstatic, ν is a speed of SAW, and c is an average speed of light in theoptical waveguide.

[0105] Accordingly, a selected wavelength difference Δλ caused bywhether the propagation directions of the light and SAW are forwarddirections or reverse directions can be represented by the followingequation (2). $\begin{matrix}{{{\Delta\lambda} = {2 \cdot \lambda_{0} \cdot \frac{v/c}{1 - \left( {v/c} \right)^{2}}}}~} & (2)\end{matrix}$

[0106] In the rejection type optical filter with three AOTFs cascadeloop connected as shown in FIG. 7, the selected wavelengths in the AOTFsat respective stages are different from one another due to the inherentwavelength deviation caused by variations in manufacturing process ofthe substrate 1 in addition to the selected wavelength difference Δλ dueto the above mentioned selected wavelength Doppler shift. The wavelengthdeviation caused by variations in manufacturing process, for example, isinherently caused in individual substrates due to manufacturing errorsin width of the optical waveguides 21 to 23 at respective stages.

[0107] For the wavelength characteristics of the rejection type opticalfilter, for example, as shown in the conceptual diagram of FIG. 13A, itis an ideal to have a filter characteristic that is changed inrectangular, namely, a change in transmissivity from the passing band tothe blocking band is steep and also the blocking band has a requiredwidth. In the multi-staged structure of AOTFs, basically, the filtercharacteristic having an excellent extinction ratio can be obtained, asthe number of stages is increased. At this time, if the selectedwavelengths at the respective stages are all coincident, as shown in theconceptual diagram of FIG. 13B, since the transmissivity becomes minimumat one point, the width of blocking band becomes narrower. For theblocking band of the rejection type optical filter, a required widthneeds to be ensured, considering the conditions of, for example, thewavelength width of optical signal corresponding to the spectrum widthof light source such as laser, errors in setting or controlling ofAOTFs, or the unstable wavelength of light source. Therefore, accordingto the filter characteristics as shown in FIG. 13B, it becomesimpossible to block the passing of optical signal of desired wavelengtheven in a case a slight variation is caused in the setting of theoptical signal wavelength or the setting of filter.

[0108] Therefore, in the rejection type optical filter shown in FIG. 7,the wavelength deviation inherent to the substrate caused by variationsin manufacturing process is considered and also the selected wavelengthdifference Δλ due to the selected wavelength Doppler shift is utilized,to ensure a required width of blocking band by slightly deviating theselected wavelengths in the AOTFs at respective stages with one anotheras shown in FIG. 13C.

[0109] Specifically, when the selected wavelengths corresponding to therespective optical waveguides 21, 22, 23 when SAWs of the same frequencyf are given in the forward directions to the propagated lights are madeλ_(1F), λ_(2F) and λ_(3F), while the selected wavelengths correspondingto the respective optical waveguides 21, 22, 23 when SAWs of the samefrequency f are given in the reverse directions to the propagated lightsare made λ_(1R), λ_(2R) and λ_(3R), there occurs various patterns in thewavelength deviation inherent to the substrate caused by variations inmanufacturing process, as shown in FIG. 14A to FIG. 14C, for example.Such wavelength deviation patterns of the three staged AOTFs can beclassified into six patterns P1 to P6 as shown in FIG. 14D when thevalues of λ_(2R)−λ_(1R) are put on the horizontal axis and the values ofλ_(3R)−λ_(1R) are put on the transverse axis with the selectedwavelength λ_(1R) as the reference.

[0110] In order to realize the selected wavelengths that are slightlydeviated among the respective stages as shown in FIG. 13C, it isrequired to determine optimum combinations of the wavelength deviationof the patterns P1 to P6, with the wavelength difference due to theselected wavelength Doppler shift. When determining the optimumcombinations, it is desired to consider the condition that theconnection relationship between the input and output for suppressing theinfluence of the stray light S as mentioned above, and the connectionrelationship in which such kinds of fiber arrays as explained in FIG.10, are satisfied simultaneously. The optimum combinations satisfyingall of these conditions can be determined corresponding to therespective patterns P1 to P6 in FIG. 13D, and the combination resultsare shown in FIG. 15.

[0111] In FIG. 15, the numerals {circle over (1)} to {circle over (6)}indicated at both ends of the substrate show the connecting orders ofAOTFs at respective stages. Further, characters such as “F-F-R”(forward-forward-reverse) indicated at the upper part of the substrateshow the propagation direction of SAW relative to the light beingpropagated through the optical waveguide positioned at the upper stageof the substrate in the figure, the propagation direction of SAWrelative to the light being propagated through the optical waveguidepositioned at the middle stage of the substrate, and the propagationdirection of SAW relative to the light being propagated through theoptical waveguide positioned at the lower stage of the substrate, inthis sequence. Further, arrangements of respective polarization axes ofwhen the respective PANDA type fibers connected to the both ends of thesubstrate are made fiber arrays of same kind, are shown on the right andleft sides of the substrate.

[0112] The constitution of the rejection type optical filter shown inFIG. 7 specifically illustrates the connection relationshipcorresponding to the pattern P1 in FIG. 15. For the selected wavelengthDoppler shift, the arrangement of the IDTs 41, 43 and 42 at therespective stages are set so that, to the light given via the connectingoptical path 2 _(B), the propagation direction of SAW in the AOTF of thefirst stage corresponding to the optical waveguide 21 is the forwarddirection, the propagation direction of SAW in the AOTF of the secondstage corresponding to the optical waveguide 23 is the reversedirection, and the propagation direction of SAW in the AOTF of the thirdstage corresponding to the optical waveguide 22 is the forwarddirection. In the AOTFs at respective stages, since the RF signal of thesame frequency is given to the IDTs, the wavelength difference due tothe selected wavelength Doppler shift corresponding to the aboveequation (2) is caused between the selected wavelengths at the first andthird stages, and the selected wavelength at the second stage. Thus, bycombining the wavelength difference with the inherent wavelengthdeviation of the pattern P1, it becomes possible to realize the filtercharacteristic as shown in FIG. 13C.

[0113] Meanwhile, in the present rejection type optical filter 1, the TEmode light given from the PBS 5 to the PBS 32 a of the substrate 1 viathe connecting optical path 2 _(C) and the polarization rotating section6 passes through the AOTFs at respective stages sequentially, in reverseto the TE mode light given to the PBS 31 a of the substrate 1 via theconnecting optical path 2 _(B), namely, passes sequentially through theoptical waveguide 22, PBS 32 a, connecting optical path 2 ₂₃, PBS 33 a,optical waveguide 23, PBS 33 b, connecting optical path 2 ₁₃, PBS 31 b,optical waveguide 21 and PBS 31 a, to be output to the connectingoptical path 2 _(B), and is returned to the PBS 5 under the polarizationstate just as is without polarization plane thereof rotated. In thisreverse propagation of the polarization light, the mode converted TMmode light corresponding to the selected wavelength when beingpropagated through the optical waveguide 22, is branched by the PBS 32 bas the dropped light, to be sent to the optical isolator 101B of thefirst monitoring section 100.

[0114] The respective polarization lights with polarization planesthereof being orthogonal to each other, returned to the PBS 5 via theconnecting optical paths 2 _(B) and 2 _(C), are multiplexed by the PBS 5and thereafter sent to the optical circulator 4 via the connectingoptical path 2 _(A), to be output to the output optical path 2 _(OUT)after passing from the port 4 b to the port 4 c.

[0115] As mentioned above, when the polarization lights from theconnecting optical paths 2 _(B) and 2 _(C) are propagated inbi-directions through the three staged AOTFs cascade loop connected onthe substrate 1, the stray light generated from each of the PBSs 31 aand 32 a at the one end of each of the optical waveguides 21 and 22 ispropagated toward the end face on the opposite side to the optical inputside of the substrate 1. However, since the connecting optical paths 2_(B) and 2 _(C) are connected to the PBSs 31 a and 32 a positioned onthe same end face of the substrate 1, respectively, the stray light ishard to be coupled to the polarization lights being propagated near thePBSs 31 a and 32 a.

[0116] Moreover, in the present rejection type optical filter, thedropped lights branched by the PBSs 31 b and 32 b, pass through theoptical isolators 101A and 101B of the monitoring section 100, to beconverted into electrical signals at the light receivers 102A and 102B,respectively, and further are added up by the circuit 103 to be sent tothe RF signal controlling section 200 as the monitor signal M. In the RFsignal controlling section 200, the peak wavelengths of the droppedlights are detected based on the monitor signal M, and an amount ofwavelength deviation to the previously set selected wavelength isobtained.

[0117] In the RF signal controlling section 200, as a method fordetecting the peak wavelengths of the dropped lights based on themonitor signal M, for example, a method to add dithering to thefrequency f of RF signal to be applied commonly to the IDTs 41 to 43 atthe respective stages, is suitable. Specifically, in a case where thefrequency f of RF signal is set to, for example, 170 MHz, 4 kHz or thelike is set as the frequency Δf of the dithering, and the RF signal ofwhich frequency fluctuates within a range of f±Δf is applied to each ofthe IDTs 41 to 43. Thus, the selected wavelengths to be mode convertedin the AOTFs at the respective stages fluctuate corresponding to thefrequency Δf of the dithering. Accordingly, the monitor signal M to bemonitored by the monitoring section 100 includes frequency componentscorresponding to the dithering. Thus, it becomes possible to detect thepeak wavelengths of the actually dropped lights by utilizing thedetected frequency components.

[0118] Here, in a case where the dithering is added to the frequency ofRF signal, it is desirable that, for the blocking band as shown in FIG.13C, the dropped light is taken out from the AOTF stage of whichselected wavelength is positioned at the center of the blocking band, tomonitor the dropped light by the monitoring section 100. This is auseful setting for realizing the stable peak wavelength detection, byavoiding such a situation where, for example, if the dropped light fromthe AOTF stage of which selected wavelength is positioned at the endportion of the blocking band is monitored, the wavelength of the droppedlight fluctuating by the dithering reaches the wavelength region wherethe transmissivity is steeply changed, so that the level of droppedlight to be monitored by the monitoring section 100 is largely changed,thereby resulting in a possibility that the peak wavelength of droppedlight cannot be accurately detected.

[0119] In the constitution of FIG. 7, the setting of the blockedwavelengths (selected wavelength) corresponding to the opticalwaveguides 21 to 23 on the substrate 1 is indicated in the relationshipas shown in FIG. 16. Therefore, for the light given to the substrate 1via the connecting optical path 2 _(B) and propagated sequentiallythrough the optical waveguides 21, 23 and 22, the monitoring isperformed on the dropped light in the optical waveguide 21 correspondingto the wavelength λ_(1F) positioned substantially at the center of theblocking band, depending on the relationship of blocking wavelength asshown by a bold line in the figure. Moreover, for the light given to thesubstrate 1 via the connecting optical path 2 _(C) and propagatedsequentially through the optical waveguides 22, 23 and 21, themonitoring is performed on the dropped light in the optical waveguide 22corresponding to the wavelength λ_(2R), depending on the relationship ofblocking wavelength as shown by a thin line in the figure.

[0120] Based on the peak wavelengths of dropped lights detected in theabove manner, the wavelength deviation amount to the previously setselected wavelength is obtained by the RF signal controlling section200, and a controlling signal for correcting the frequency of RF signalis generated according to the wavelength deviation amount, to be outputto the RF signal generating circuit 40. Then, in the RF signalgenerating circuit 40, in accordance with the controlling signal fromthe RF signal controlling section 200, the frequency f of RF signal iscorrected, and the corrected RF signal is applied commonly to the IDTs41 to 43 at the respective stages. Thus, even if the filtercharacteristic is changed due to a change in temperature, deteriorationwith time lapse or the like, it becomes possible to block reliably andstably a light desired wavelength from passing through, by tracking andcontrolling the frequency of RF signal.

[0121] As described above, according to the present rejection typeoptical filter, it is possible to suppress a leakage phenomenon of straylight being propagated within the substrate 1, to the optical outputside, and to effect the individual characteristics of three AOTFs on thesame substrate 1. Thus, it becomes possible to easily realize arejection type optical filter having an excellent filter characteristicin which the extinction ratio exceeds 40 dB.

What is claimed is:
 1. A connecting method of optical function devices,for connecting a plurality of optical function devices formed on thesame substrate, wherein said plurality of optical function devices arecascade connected, so that both ends of an optical path passing throughsaid plurality of optical function devices are positioned on the sameend face of said substrate.
 2. A connecting method of optical functiondevices according to claim 1, wherein both ends of said optical pathpassing through said plurality of optical function devices are connectedto each other, to be in a cascade loop connection, so that a light isinput/output via said connected portion.
 3. A connecting method ofoptical function devices, for connecting a plurality of optical functiondevices formed on the same substrate, wherein said plurality of opticalfunction devices are connected in parallel, so that light input/outputdirections of said optical function devices adjacent to each other onsaid substrate are opposite to each other.
 4. An optical apparatuscomprising: a plurality of optical function devices formed on the samesubstrate; and a cascade connecting section that cascade connects saidplurality of optical function devices so that both ends of an opticalpath passing through said plurality of optical function devices arepositioned on the same end face of said substrate.
 5. An opticalapparatus according to claim 4, further comprising; a loop connectingsection that connects both ends of said optical path passing throughsaid plurality of optical function devices, wherein a light isinput/output to/from said loop connecting section.
 6. An opticalapparatus according to claim 4, wherein said plurality of opticalfunction devices includes an optical switch device.
 7. An opticalapparatus according to claim 4, wherein said plurality of opticalfunction devices includes an optical filter device.
 8. An opticalapparatus according to claim 7, wherein said optical filter device is anacousto-optic tunable filter (AOTF).
 9. An optical apparatus accordingto claim 4, wherein said plurality of optical function devices includesan optical waveguide type device.
 10. An optical apparatus, comprising:a plurality of optical function devices formed on the same substrate; aparallel connecting section that connects said plurality of opticalfunction devices in parallel, so that light input/output directions ofoptical function devices adjacent to each other on said substrate areopposite to each other.
 11. An optical apparatus according to claim 10,wherein said plurality of optical function devices includes an opticalswitch device.
 12. An optical apparatus according to claim 10, whereinsaid plurality of optical function devices includes an optical filterdevice.
 13. An optical apparatus according to claim 12, wherein saidoptical filter device is an acousto-optic tunable filter (AOTF).
 14. Anoptical apparatus according to claim 10, wherein said plurality ofoptical function devices includes an optical waveguide type device.